3 mol % yttria (Y2O3)-stabilized zirconia (tetragonal zirconia polycrystal, which is hereinafter referred to as “3Y-TZP”), manufactured by a sintering process, is known to have excellent physical properties including hardness, flexural strength, fracture toughness, etc., but suffers from severe degradation properties, in which the strength of the sintered zirconia is drastically decreased due to microcracking therein, when the sintered zirconia is allowed to stand at a low temperature of about 200° C. in a humid atmosphere for several hours. Such degradation properties are referred to as low-temperature degradation, the causes of which have not yet been clearly determined, and are merely based on hypotheses put forth by Yoshimura, Lange, Weinnubst, Burggraal, Azzoni, Kruse, and Simon Lawson. With the goal of solving low-temperature degradation problems, methods of adding a small amount of material such as Mn, Ge or Ce or further fining grains of zirconia, serving as a main ingredient, have been introduced through a number of experiments. However, these methods have served only to slightly increase resistance to low-temperature degradation, and thus fundamental solutions are still required. Korean Patent Publication Application No. 10-2001-0041132 discloses a method of preventing the appearance of low-temperature degradation in zirconia-containing material.
3Y-TZP, which was published on Nature by Dr. Garvie, 1975, has been applied to human artificial joint surgery in Europe since the 1980s depending on only the superior physical properties without knowing low-temperature degradation thereof. However, there have occurred serious medical emergencies due to 3Y-TZP. Specifically, significant defects, including the breaking of operated joints because of the reduced strength attributable to the low-temperature degradation of 3Y-TZP, have occurred, and thus the use of sintered zirconia (3Y-TZP) in human bodies was banned by the FDA (USA) in 2001, and reoperation of already-operated artificial joints using titanium metal was ordered.